FIG. 1 is a simplified diagram illustrating a portion 10 of the Earth's surface, a horizon 12, and first and second land masses 14 and 16. A hostile missile 18 has been launched from land mass 14 and has taken a path 20 to its current location. A friendly ship 22 bears a radar system designated generally as 24. It will be understood that the radar system 24 may as well be land-based rather than ship-based. Radar 24 senses missile 18 by means of electromagnetic radiation illustrated by a conventional “lightning bolt” symbol 26.
FIG. 2 is a simplified block diagram illustrating a radar system 210 which may be used in radar 24 of FIG. 1. In FIG. 2, an antenna illustrated as 212 is connected to a transmit/receive element (T/R) illustrated as a block 214. A transmitter (TX) 216 is coupled to the transmit input port 214t of T/R 214, and a receiver (RX) illustrated as a block 218 is connected to receive signal port 214r of T/R 214. A radar controller 220 at least synchronizes the transmitter and receiver, all as well known in the art.
The received signals from receiver 218 are coupled by way of a path 221 to a tracking processor 222. Processor 222 processes the measurements of the target, and generates time-sequences of estimated target location, which are known as tracks or target tracks. The target tracks are continuously updated with current measurements and extrapolated to estimated future locations of the target. The results of this processing are estimated future target states and covariance. The estimated future target states and covariance are provided by way of a path 223 to a fire control system illustrated as a block 224. Fire control block 224 selects countermeasures appropriate to the threat and its state, and initiates the countermeasures, as by initiating control of an interceptor missile. The fire control information is provided to an interceptor missile and/or its controller, as suggested by block 226. An appropriate countermeasure is made, as by launch of an interceptor missile 28 of FIG. 1 and guidance of the interceptor to an intercept point 30 with the hostile missile 18.
The system of FIGS. 1 and 2 relies on a stream of measurement data from the sensors, which in the illustrated scenario includes a radar system. The measurement data is a stream or time sequence (t1, t2, . . . , tN−1, tN, tN+1, . . . tK) of range, range-rate, and azimuth and elevation angle measurements. The stream of data is subject to dropouts and the effects of target maneuvers, which degrade the performance of the various filters used to estimate the target location, and also tends to degrade the discrimination among various types of objects being tracked. This is particularly true in those cases in which the target separates into multiple objects, such as booster, decoys, and payload.
FIG. 3 is a simplified block diagram 310 of a prior-art tracking processor 222 of FIG. 2. In FIG. 3, the received measurement is applied from path 221 in common for in parallel) to range-state filter 312 and to a Cartesian coordinate converter illustrated as a block 314. Range-state {right arrow over (r)} includes r, {dot over (r)}, and {umlaut over (r)} that is range, and its time derivatives. Converter 314 converts the measurement data, expressed in terms of range, range rate, azimuth angle, and elevation angle, into (ship-based) Cartesian X,Y,Z coordinates using well-known spherical-coordinate-to-Cartesian-coordinate transformations. The Cartesian-coordinate information is also termed “3-D” information. The coordinate-converted measurement data is applied to a Cartesian Kalman filter block 316, which forms and updates tracks based upon the flow of measurement data, to produce Cartesian states and covariance. More particularly, Cartesian Kalman filter 316 uses Kalman filtering techniques to smooth Cartesian X, Y, Z measurements into X, Y, Z, Xvelocity, Yvelocity, Zvelocity states. Propagation portions of the Kalman filter apply the effects of gravity to the states. The filtered Cartesian states and covariance data from block 316 are applied by way of path portion 315 to output path 223, to a centrifugal determination block 318, and to a target discrimination block 320. Block 318 uses the track data to estimate the centrifugal acceleration of the target(s). The centrifugal acceleration information is used inter alia for updating the range rate.
Range-state filter 312 of FIG. 3 receives the centrifugal acceleration information from block 318 together with the measurement data from path 221, and updates the filter range state. That is, range-state filter block 312 provides information about target range, target range rate, and target acceleration. The range-states provided by range-state filter 312 are provided to target discriminator block 320 and by way of a portion 313 of path 223 to the fire-control block 224 of FIG. 2. Thus, target discriminator block 320 of FIG. 3 receives the target range, target range rate, and target acceleration from block 312, and also receives the track state information from block 316. Discrimination block 320 produces object identification information. The object identification information from block 320 is coupled by path 225 to countermeasure fire control system 224 of FIG. 2. The target state and covariance information are applied from blocks 312 and 316 over portions 313 and 315 of path 223 to fire control system 224 of FIG. 2. Ultimately, this results in initiating interceptor missile (or other countermeasure) control for engaging the hostile missile.